In the electronics industry, testing of circuit boards is done routinely to verify that the board, when populated with components, functions properly. Such testing is usually accomplished by placing the circuit board on a test fixture having a plurality of double-ended pins, each having a first end adapted to contact a selected node on the undersurface of the circuit board. The other end of each pin is electrically connected to an automatic testing machine, known in the art, which tests a circuit board by launching test signals therein via the test fixture and then sensing the signals returned from the board in response to the test signals. By monitoring the level of the signals returned from the circuit board, an indication can be had as to whether the circuit board is functioning properly.
Proper testing of a circuit board demands that there be a solid electrical connection between the nodes on the circuit board and the pins of the test fixture. Otherwise, proper communication of signals between the circuit board and the testing machine will not occur. The lack of a good connection between even one of the pins on the test fixture and one of the nodes on the circuit board may cause a good board to be found faulty, or worse, a defective board to be found good.
In the past, verification of continuity between the circuit board and the test fixture has been accomplished by injecting a dc voltage into one of a pair of test fixture pins while the other pin of the pair is grounded. When each of the pair of pins contacts a separate one of a pair of nodes on the circuit board connected to each other by a circuit path created by one or more components on the board, then current passes between the pins. The presence of a current passing between the two pins indicates that each pin of the pair is in contact with a corresponding one of a pair of nodes on the circuit board. Continuity between every other pair of test fixture pins and pair of corresponding nodes on the circuit board is tested in the same way.
Unfortunately, the above approach does not always yield accurate results. Even when each of a pair of test fixture pins contacts a separate one of a pair of nodes, very little current may pass between the pins if the nodes on the circuit board are interconnected by one or more very high impedance components. Thus, the absence of a meaningful current passing between a pair of pins will indicate the lack of continuity between the pins and the nodes on the circuit board even when such is not the case.
Therefore, a technique is needed for correctly sensing whether each of a pair of pins of a test fixture is in contact with each of a pair of nodes on a circuit board even when the nodes are interconnected by a high impedance circuit path.